A capacitor with a variable capacitance, used in a miniaturized accelerometer or a gyrometer, comprises a mobile comb and a fixed comb. The teeth of these combs are interdigital and together form a capacitance whereof the air-gap varies according to the movement of the mobile comb in a direction perpendicular to the main axis, or “longitudinal” axis, of the teeth, and whereof the opposite surfaces have a constant area. A measurement and control device is moreover connected to the mobile and fixed combs to measure the capacitance that they form together and, by means of a feedback loop, to control the position of the mobile comb at a central position.
It is commonly observed that each of the teeth of the mobile comb has a natural vibration mode and is capable of resonating with the neighboring teeth when the teeth of the mobile comb are subject to a movement in a direction perpendicular to their axis (referred to as the transverse direction). When the teeth of the mobile comb vibrate together (referred to as “transverse resonance mode” of the mobile comb), the capacitor often becomes unusable due to the noise generated by this vibration. Moreover, the position control of the mobile comb may become unstable, and the teeth are weakened due to the high bending forces that they undergo.
To overcome such drawbacks, documents FR-A-2 858 853 and FR-A-2 858 854 disclose combs of which the teeth, all identical, have a cross section of variable width along their longitudinal axis, and a particular choice of a closed loop for position control. The transverse resonance mode of the mobile comb is thus positioned in a zone characterized by a substantially zero gain of the closed loop. Through an appropriate choice thereof, this zone is furthermore located outside the range of useful frequencies of the capacitor, so that the entry of the teeth of the mobile comb into resonance does not significantly disturb the measurement of the capacitance formed by the combs.
However, since the teeth of the mobile comb are always identical, the transverse resonance mode of the mobile comb is not eliminated as such. The teeth of the mobile comb are therefore still subject to high bending forces that may weaken them. Moreover, the geometry proposed by the documents mentioned above gives rise to larger teeth and consequently a lower grade capacitor.